This invention relates to electric power inverters for converting direct current (d-c) to polyphase alternating current (a-c), and more particularly it relates to improvements in the old and well known current-fed "third harmonic" auxiliary impulse commutated inverters. The principles of commutation and a typical practical application of such inverters were described in a technical paper entitled: "Analysis of a Novel Forced-Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive," which paper was presented by R. L. Steigerwald and T. A. Lipo at the IEEE/IAS annual meeting held in Los Angeles, Calif. on Oct. 2-4, 1977. The Steigerwald and Lipo paper was reprinted in IEEE TRANS. Vol. IA-15, No. 1, January/February 1979, pgs. 14-24, and it is expressly incorporated herein by reference.
In essence, a third harmonic auxiliary impulse commutated inverter comprises six main unidirectional conduction controllable electric valves, such as thyristors, that are interconnected in pairs of series aiding, alternately conducting valves to form a conventional 3-phase, double-way, 6-pulse bridge between a pair of d-c terminals and a set of three a-c terminals. The d-c terminals of the bridge are adapted to be connected to a suitable source of relatively smooth direct current. A large, multicell, heavy duty electric storage battery is a suitable current source, as is the combination of an electric power rectifier to which an alternating voltage source is connected and a current smoothing reactor or choke in the d-c link between the d-c terminals of the rectifier and inverter, respectively. The a-c terminals of the aforesaid bridge are respectively connected to the different phases of a 3-phase electric load circuit which typically comprises star-connected 3-phase stator windings of a dynamoelectric machine such as a large synchronous motor.
To supply the load circuit with 3-phase alternating current, the six main valves of the inverter are cyclically turned on (i.e., rendered conductive) in a predetermined sequence in response to a family of "firing" signals (gate pulses) that are periodically generated in a prescribed pattern and at desired moments of time by associated control means. To periodically turn off the main valves by forced commutation, the inverter is provided with an auxiliary circuit comprising a precharged commutation capacitor and at least seventh and eighth alternately conducting unidirectional controllable electric valves that are arranged to connect the capacitor between the neutral or common point of the 3-phase a-c load circuit and either one of the d-c terminals of the bridge.
During each full cycle of steady state operation of a third harmonic inverter, each of the valves in the auxiliary commutation circuit is briefly turned on three separate times. More particularly, the 7th valve is fired at intervals of approximately 120 electrical degrees, and the 8th valve is fired at similar intervals that are staggered with respect to the intervals of the 7th valve, whereby one or the other auxiliary valve is fired every 60 electrical degrees. When an auxiliary valve is turned on, the commutation capacitor is effectively placed in parallel with one phase of the load circuit and a first one of the two main valves which are then conducting load current. Initially, the capacitor voltage magnitude is higher than the amplitude of the line-to-neutral voltage that is developed across the inductive load, and its polarity is such that the capacitor starts discharging. Consequently current is forced to transfer (commutate) from the first main valve (i.e., the offgoing or relieved valve) to a parallel path including the turned-on auxiliary valve and capacitor. The rate of change of current during commutation will be limited by the load inductance.
After current in the offgoing main valve decreases to zero, the magnitude of capacitor voltage is still sufficient to keep that valve reverse biased for longer than its "turn-off time." As soon as the commutation capacitor is fully discharged, load current begins recharging it with opposite polarity. Once the commutation capacitor is recharged to a voltage magnitude exceeding that of the line-to-neutral load voltage, the next (oncoming) main valve in the bridge is forward biased and can be turned on, whereupon load current commutates from the turned-on auxiliary valve and commutation capacitor to the oncoming main valve. This causes the auxiliary valve to turn off and completes the commutation process. The capacitor is left with voltage of proper polarity and sufficient peak magnitude for successful commutation of the second one of the first-mentioned two conducting main valves when the opposite auxiliary valve is turned on approximately 60 degrees later. It will be apparent that there are six intervals of commutation per cycle, the direction of current in the commutation capacitor during each interval is reversed compared to the preceding interval, and therefore the fundamental frequency of the alternating capacitor current equals the third harmonic component of load frequency.
For successful operation, a third harmonic auxiliary impulse commutated inverter needs to include suitable means for charging the commutation capacitor before normal operation commences. Heretofore, the capacitor has been precharged either through a resistor from an auxiliary power supply or, as suggested by Steigerwald and Lipo, by simultaneously turning on an auxiliary valve and an opposite one of the main valves to provide a path for capacitor charging current from the electric power source to which the d-c terminals of the inverter are connected. The latter path, which is a resonant circuit including the inductance of the reactor in the d-c link and the inductance of one phase of the load circuit in series with the capacitance of the commutation capacitor, will conduct a single pulse of current until the capacitor charges to a peak voltage approaching twice the magnitude of source voltage, whereupon the current falls to zero and both of the conducting valves are self commutated by the resonant circuit to their turned off (non-conducting) states. Once the third harmonic commutation mode of operation commences, the precharged capacitor can be "pumped up" to an even higher predetermined voltage level.
As is pointed out in the referenced Steigerwald and Lipo paper, one practical application of a current-fed third harmonic auxiliary commutated inverter is in an adjustable speed a-c drive system where the 3-phase star-connected stator windings of a rotatable synchronous machine are supplied with variable frequency a-c power by the inverter which needs to be forced commutated in order to start the machine. Such a machine can advantageously be used to start or "crank" a prime mover such as a large internal-combustion engine. For this purpose the rotor of the machine is coupled to a mechanical load comprising the crankshaft of the engine. Initially the output torque of the rotor (and hence the magnitude of current in the stator windings) needs to be relatively high in order to start turning the crankshaft. As the rotor accelerates from rest, less torque (and current) will be required, while the fundamental frequency of load current increases with speed (revolutions per minute). In its cranking mode of operation, the inverter supplies the machine with current of properly varying magnitude and frequency until the engine crankshaft is rotating at a rate that equals or exceeds the minimum speed at which normal running conditions of the engine can be sustained.